Humidity variable resistance



June 9, 1942. F. w.`DuNMoRr- HUMIDITY VARIABLE RESISTANCE Filed June 8, 1940 2 Sheets-Sheet l 47/ l l onli/.ed

INVENTOR ATTORNEY June 9, 1942. F. w. DUNMORE HUMIDITY VARIABLE RESISTANCE Filed June 8, 1940 2 Sheets-Sheet 2 mmmmwmm INVENTOR ATTORNEY Patented June 9, 1942 I-IUIVIIDITY VARIABLE RESISTAN CE Francis W. Dunmore, Washington, D. C., assignor to Government of the United States, as represented by the Secretary of Commerce, and his successors in oilce Application June 8, 1940, Serial No. 339,530

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 16 Claims.

The invention herein disclosed may be used by or for the Government of the United States for governmental purposes without the payment of any royalty therefor.

The wet and dry bulb method of measuring humidity has long been a common means of determining the moisture content ofair. There are, however, many circumstances in which this method is not applicable, a particular case being the measurement of upper-air humidities by means of the radio sonde. In fact in all cases where humidity readings are to be read remote from the point of measurement, recorded graphically, or where measurements must be made in small conned spaces or at low temperatures, the wet and dry bulb method is not practical.

Attempts have been made to use the hair-type hygrometer but its slow rate of response and other peculiarities have made its use undesirable in many incidences. In radio meteorography the large time lag in response of the hair hygrometer introduces serious errors, since sudden humidity changes are often encountered by a rapidly ascending balloon. This lag increases with decreasing temperature so the accuracy of this type of hygrometer becomes progressively less with increasing altitude.

In my earlier application, Ser. No. 247,243, filed December 22, 1938, an electrical type of hygrometer is disclosed which is particularly adapted for use with the radio sonde, and which, used with certain precautions, more nearly satisfies all the requirements for a remote humidity indicating device without moving parts or appreciable lag. The form therein disclosed consisted of two fine tinned copper wires spirally wound simultaneously on a thin-walled etched glass tube so as to form 20 dual turns per inch. After winding, the glass tube was preferably coated with a dilute solution of lithium chloride in water. The lithium with the moisture which it takes up from the air formed a resistor on the glass surface between the two wires which varied greatly with the moisture content of the air so that the resistance of the unit could be expressed in terms of percent relative humidity, Such a unit gave satisfactory results when certain precautions were observed in its use. These precautions were necessary because of a continuous short and long period aging effect, and a polarization error when used in direct-current circuits. It was also found necessary to switch in two or more units of different sensitivity in order to cover the full humidity scale.

The present invention relates to such an electrical hygrometer and aims generally to improve the same. This invention aims in particular to overcome the above mentioned difilculties, severally, and collectively, and to produce a unit particularly, but not exclusively, suitable for radio sonde use, and which also has advantages over the wet and dry bulb method under many other conditions of measurement. It makes possible more reliable humidity readings, graphically recorded if desired, remote from the point of measurements, as well as more accurate measurements in confined and inaccessible spaces. The original features of rapid rate of response and inherent variation in resistance with humidity have been retained.

In the accompanying drawings illustrative of the improvements of the present invention- Figs. 1 and 1a are side and end elevations, respectively, of a unit embodying features of the invention;

Figs. 2 and 3 are diagrammatic showings of preferred multiple unit arrangements according to the invention;

Fig. 4 is a diagram showing one form of connection for alternately transmitting humidity and temperature indicating factors;

Fig. 5 is a representative graph for evaluating true humidity from observed humidity and temperature;

Fig. 6 is a graph illustrating frequency-observed humidity characteristics of the arrangement of Fig. 3;

Fig. '7 is a comparative flight record of humidity and temperature plotted against altitude, showing in solid lines the rapid response of the device of the present invention as compared with the response of a concurrently used hair hygrometer, shown in dotted lines.

Fig. 8 is a further chart showing relative speed of response of a hair hygrometer (dotted line) and an electric hygrometer according to this invention (solid line) at 0 C. when taken from air at 30% relative humidity and put in a 500 ft. per minute air stream at 66% relative humidity.

Fig. 8a is a similar chart showing relative speed of a hair hygrometer (dotted line) and an electric hygrometer according to this invention (solid line) at 24 C. when taken from air at 68% relative humidity and put in a 500 ft. per minute air stream at 35% relative humidity.

Fig. 9 shows diagrammatically a preferred arrangement for alternating current operation of the device.

Fig. 10 is a humidity-resistance graph for five individual units A, B, C, D and E, at 24 C., coated with 6:1 polyvinyl acetate containing 0.0%; 0.25%; 0.5%; 1.0% and, 2.2% lithium chloride solution, respectively.

Fig. 11 is a humidity-resistance graph at 24 C. of the five element electric hygrometer cir' cuit of Fig. 2, with R1, R. R3, R4 and R5 valued at 0.075, 0.15, 0.35, 1.0 and 3.0 megohms respectively; and with units A, B, C, D and E having the same values as in Fig. 10.

Fig. 12 illustrates a multi-scale arrangement for theindicating meter enabling direct reading of relative humidity corrected for any temperature between +30 and 30 C.,

Fig. 13 shows a multi-scale arrangement for use at room temperature, using five electric hygrometer units (which may be valued as in Fig. 10 and connected as in Fig. 9) each covering a different part of the humidity range, and

Fig. 14 is a block diagram of an oscillator powered portable device embodying features of this invention.

Certain of what are now regarded as the more important improvements aiiected by various features of the present invention may be classified under five headings: (l) prevention of longperiod aging: (2) prevention of a hysteresis effect or short-period aging; (3) discovery of the advantages of using partially hydrolyzed polyvinyl acetate film, (a) as a binder for hygroscopic salt, (b) in producing a homogeneous film; (c) for reduction of polarization; (4) improved structure and methods of obtaining same; and.

() special provisions for covering the full humidity range preferably without switching means.

1. PaEvEN'rING LONG-PERIOD AGING Research work with Athe dual-coil electric hygrometer described in my above identified application has shown that the electrical resistance of the unit therein described continued to increase from Week to week so that it was n'ecessary to calibrate the device shortly ibefore using. It has since been found that this increase in resistance was due to some form of film which gradually formed on the surface of the wire used for the dual coil. After winding units with many kinds of wire, it has now been discovered that this aging effect can be eliminated by employing platinum or palladium wire. As palladium is cheaper than platinum the former is ipreferred. Twenty dual turns per inch of No. 38 American wire gauge appear to give the best results. Any fewer turns per inch increases the current density at the surface of the Wire which tends to increase the polarization effect in direct-current circuits as well as to decrease the life of the unit in alternating current circuits.

2. ELIMINATION or SHORT-PERIOD AGING Another phenomenon termed hysteresis or short period aging also made it necessary to calibrate the units of my prior application shortly before using, or to store them'at a fixed humidity. The cause of this eect has now been discovered to be the 'adsorption of water by the etched glass surface upon which the dual coils were wound, and to which the lithium chloride coating was applied. As this giving up or taking on of water by the glass required several hours the result was a shift in the calibration curve for the unit to lower values of resistance if the unit had been exposed to a high humidity for several hours or to higher resistance v-alues if the unit had been exposed to a low humidity for several hours.

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The only way to overcome this effect in the units described in my previous application was to store the units in a fixed humidity chamber until ready for use.

After an exhaustive study of many water-resistant coatings on the glass, one coating material has now been discovered which overcomes this troublesome effect; namely, DOlystyrene resin thermoplastic material. This material is sold by the Bakelite Corporation, under the trade designation XMS10023," and by the Advance Solvents & Chemical Corporation under the tradename Resoglaz. A coating of this material is applied to the glass or other supporting surface by dissolving the polystyrene in one of its solvents with some plasticizer and dipping the supporting surface in this solution. Or polystyrene may now be obtained in film form 1 mil. thick which may be wrapped 3 or 4 times around the supporting surface.

The construction now preferred, however, consists of a thin-walled aluminum tube in place of the glass tube. The yaluminum tubing is dipped in the polystyrene solution or is wrapped with 4 turns of the polystyrene film and then wound with the bifilar coil or palladium wire. The aluminum tubing is preferably of the commercial hard tempered type with an outside diameter of inch' and a wall thickness of, say 0.01 inch. A thin wall is necessary as the unit must maintain the temperature oi the air surrounding it in order to register the true relative humidity. This is especially important in radio sonde use Where sharp temperature inversions are often encountered. Such a unit is shown in Figs. 1 and la, described hereinafter.

The ,polystyrene film on the aluminum tube thus serves as a surface of high electric resistance for the bifilar coil and the moisture-sensitive coating, without influencing the functioning of the device.

In applying this film it is important that the polystyrene solution is such that the film will dry without cracking or crazing. Any such surface irregularities tend to produce a hysteresis effect or lag in response of the device. Also such a. surface structure causes non-uniformity of the coating of the moisture sensitive film which is applied after the unit is wound. If a polystyrene film is used in sheet form and wrapped around the aluminum tube it is important that this process does not produce cracking or crazing in the film.

A satisfactory polystyrene solution for coating the aluminum tubing by dipping may be made up by dissolving about 4 to 6 parts by weight of polystyrene (Resoglaz") with about 1 part by weight of plasticizer such as dibutyl phthalate, in a suitable solvent, preferably consisting of about 4 to 6 parts by weight each of xylene'and ethyl acetate mixed with about 7 to 8 parts each of toluene and butyl acetate.

A suitable quantity of solution for laboratory use has been made up of:

Grams Polystyrene (Resoglaz) 100.0 Dibutyl phthalate (plasticizer) 18.4 Ethyl acetate 90.0 Butyl acetate 133.0 Xylene '90.6 Toluene 135.0

In coating with this solution the aluminum tubing (with ends closed) was dipped inthe solution and immediately withdrawn at a steady rate of asomar approximately 1 inch per 'I seconds. 'I'his process was repeated with a drying interval between each coat of 3 hours until the nlm of styrene resin was 0.01 inch thick. After the last coat the units were heated at 80 C. for 18 hours before winding. The polystyrene surface was not, and should not be roughened as such treatment produces units which are not reproducible when coated with the moisture sensitive 111m.

3. DrscovEsY or ADvAN'rAaas or UsINo 'PAa'rrALLY HYnaoLYzEn PoLYvINYr. Aoars'rn FILM a. Use as a binder jor the lithium chloride The moisture-sensitive material used on the polystyrene surface in the lower ranges of humidity comprises a suitable salt solution, preferably a dilute solution of lithium chloride in water. In order to keep the lithium chloride on the smooth polystyrene surface, a suitable binder had to be found for it which would mix readily with the lithium chloride water solution and form a thin film which' would readily pass water vapor without producing appreciable lag in the response of the unit.

During the work on the electrical hygrometer search was conducted for various hygroscopic materials and revealed that a polyvinyl acetate called Hydrogel or Solvar" was somewhat hygroscopic. Such material was accordingly tried (without lithium chloride) as a sensitive hygroscopic coating on shellac spar varnish and balata as bases. These combinations were not well suited for radiometeorological use. In connection with the present invention it has been discovered that this material carried by a polystyrene surface, particularly when mixed with lithium chloride, produces a most satisfactory sensitive element. When applied to the polystyrene resin surface the lithium chloride provides the water-vapor sensitive element and the polyvinyl acetate film thel hygroscopic binder gan Products Corporation, Empire State Bldg.,4

New York city, under the name of Solvar sample 6341 S. O. 1232, W. O. 5356-11 Reissue Patent #20,430. A proper concentration of this material was determined to be 1 part of it by volume to 6 parts (by volume) of a mixture of equal parts of water and ethyl alcohol. To this l-to-6 solution the saturated solution of lithium chloride in water (at 21 C.) is added in percentages by volume depending upon the range in the humidity scale to be covered by the unit to be coated. If used frequently, new solutions should be made each month as the evaporation of the alcohol causes the solution to become more concentrated. IThe percentages deemed particularly desirable are discussed more fully hereinafter.

b. In producing a homogeneous film Another advantage in the use of the polyvinyl acetate-lithium chloride combination is that the polyvinyl acetate runs evenly on the smooth polystyrene resin surface. Thereby when combined with the lithium chloride it effects uniform distribution thereof, as well. Lithium chloride and water alone form small globules. With the uniform lm, units may be duplicated so that they will agree in their humidity indications to within i2%. A satisfactory method of coating is to dip the wound units into the polyvinyl acetate-lithium chloride solution and withdraw them at a steady rate of 1 inch in 12 minutes.

c. For reduction of polarization A third advantage m the use or the polyvinyl acetate film proved to be a reduction of the po- 20, pages 367-392 (1938) Research Paper R. P.

1082. In normal use alternating current may be applied to the units when making `measurements in which case polarization errors are not present.

However, it is necessary to use the same type of circuit or indicating instrument as was used in calibrating the umts as the resistance of the unit is somewhat a function of the current flowing through it. The unit behaves a little like an electrolyte.

The reason for the reduction of polarization by the polyvinyl acetate nlm is not fully understood but since the film is more like a solid than a liquid its electrical conduction characteristics are perhaps more like those of a solid.

4. IMPaovEp S'mUcrUaE AND METHODS or Paonucmo SAME A typical single unit according to the invention shown in Figs. 1 and la may consist of a winding I0 of, say, 32 inches (about 25 bifllar turns) ci No. 38 (American wire gauge) bare soft-drawn palladium wire (ultimate tensile strength 600 grams) wound 20 bilar turns per inch on a polystyrene-coated aluminum tube I I. The tubing may be about 1% inches long with an outside diameter of inch and a wall thickness of 0.01 inch. The devices may be provided with insulated end washers I2 at one end, serving, as by solder eyelets Il, to hold the terminal leads I5, while the washers I3 at the other end serve to anchor the free ends of the dual coil I0, which in the embodiment shown are twisted about ears I3 formed on washers I3. These washers I2 and I3 are preferably made of polystyrene resin and are preferably applied over the polystyrene resin coat and anchored, as with a drop or two of the polystyrene solution, about 5/8 inch from the ends of the tube I I. This gives a longer leakage path from the terminals to the aluminum tubing. Where suiilcient rigidity can be obtained with a unit moulded from polystyrene resin with a wall thickness of 0.02 inch, then this invention contemplates that the whole unit, washers and all, may be moulded in one piece without the need for the aluminum tubing. As high temperatures might cause such a unit to lose shape and thereby loosen the winding, the formation of the device on an aluminum tube is preferred for general use.

In order to insure a tight winding the palladium wires in accordance with this invention are heated as they go on to the tubes so that they seat just slightly into the styrene resin surface.

In order to obtain units with predetermined operating characteristics, it is important that a standard method of applying the polyvinyl-acetate-lithium-chloride coating be followed. A satisfactory procedure is to mount the units with their longest axis vertical. They are quickly immersed up to the terminal washer in the solution and then withdrawn by a motor-driven device at a constant rate of 1 inch in 12 minutes. The surplus liquid clinging to the lower end of the unit is absorbed with a blotter, and the units are allowed to dry in still air.

After the units are coated, they must go through an aging period of at least 10 to 14 days before they are ready for use. During this time the resistance of the units continues to decrease until it settles down to ay iinal value (for any given humidity) where it remains for many months. Recent experiments on 19 units seem to indicate that this aging period may be reduced to 2 days by putting the units in a humidity chamber with air at 60-percent relative humidity and 26 C. and continually circulating the air gently over the units.

5. SPECIAL PsovrsroNs ron CovEaINo- THE FULL HU- mnrrr RANGE WITHOUT SwI'roHrNo MEANS If an attempt were made to cover the full humidity range with a single unit such as shown in Fig. 1, the variation in resistance of the unit would be so extreme that it would extend outside the normal limits of a measuring device. Furthermore, the excessive current through the unit at high humidities might be harmful to the unit.

In the case f the radio sonde the resistance variation for a change in humidity from to 100% from +30 C. to 30 C. should preferably be kept within the limits of 3 megohms to 10,000 ohms. When used in an A.C. circuit such as with a Weston model '164 microfarad-meter (calibrated to read in ohms) the resistance variation for a change from 5 to 100% humidity should be confined within the limits from 8 megohms to '75,000 ohms.

In order to make possible this condition my invention contemplates the provision of a multiple unit having different percentages of LiCl coatings on each of its elements. Thus by arranging a number of units in parallel, with the proper resistors in series with each, it is possible to confine the resistance variation of the composite unit to the desired limits.

Such an arrangement is shown in Fig. 2, a iive unit embodiment, in which unit A may have a polyvinyl-acetate coating only; unit B, a coating of 0.1% LiCl; C, 0.5% LiCl; D, 1.0% LiCl; E, 2.0% LiCl. Resistance R5 in this embodiment is greater than R4; resistance R4 is greater than Re, etc. At the high humidities units B, C, D and E, although practically shorted, are prevented from causing a short at terminals F by resistors Rz, Ri, R4 and Ra yet the resultant resistance at E will vary below the values of R2, Rs, R4 and Rs since unit A has resistor R1 only in series with it. Thus A is the controlling unit at high humidities. 0n the other hand, at low humidities A, B, C and D are practically open-circuited while R1, R2, Re, R4 and R5 are not suiiiciently high to prevent the change in resistance of unit D from eifecting the overall resistance at F. Thus E is the controly.

iing unit at low humidities. In a like manner units B, C and D become the controlling unit in turn at intermediate humidities. 'I'hese operating characteristics are explained more fully hereinafter.

OPERATING CHARACTERISTICS IN DIREC I- v CURRENT RADIO-SONDE CIRCUITS As the radio-sonde circuits pass pulsating direct current through the electric hygrometer, the characteristics of the unit in such a circuit diifer somewhat from those obtained when it is used in alternating current circuits. Some polarization exists in direct current circuits, but it is a fixed factor in a given type of circuit and doesnot change the calibration of the unit appreciably during the period of a flight (about 1 hour). The unit appears to have a higher resistance at a given humidity than if measured with alternating current flowing through it as described herematter.

1. Tasas-Emimm UNIT For use in the radio-sonde circuit developed at the Bureau of Standards (see J. Research N. B. S. vol. 20, R. P, 1082), a three-element unit has been found satisfactory. This unit, illustrated in Fig. 3, consists of three elements constructed as outlined above. The elements are coated with a 1%, 2% and 3% lithium chloride-polyvinyl acetate solution respectively. The three-elements are connected in parallel with 32,500 ohms in series with the2% and 3% units. and an additional 32,500-ohm resistor in series with the 3% unit.

In radiometeorograph work, the electric hygrometer and an electrolytic thermometer are alternately connected to the radio-sonde circuit in any suitable way. Desirably, this may be effected by means of a single-pole. double-throw relay which is operated in any suitable manner, as by an atmospheric pressure operated switch, traveling across a multi-segment commutator, as shown in Fig. 4, in which the expansible bellows or Sylphon I3 moves switch 20 across conducting and non-conducting segments of commutator 2i, making and breaking the circuit of relay switch 22 and battery 23. When the relay circuit is open the multi-unit humidity responsive system 24 is connected in the control circuit of fre- I quency generator 25; and when the relay circuit is closed the temperature element 26 is substituted to provide the temperature factor for evaluation of relative humidity.

2. FREQUENCY-Humour CnAaAc'rEaIs'rio graph in this ilgure is for the composite arrange- The electric hygrometer functions like an electrolyte in that its resistance increases with a decrease in temperature. In radio-sonde use where temperature variations of are encountered it is of course necessary to apply a correction to obtain the true humidity. It has not been found possible, even with a composite 3element unit, to cover the full 10-100% humidity scale, over this extreme temperature range, but it has been feasible, with the composite unit shown in Figs. 3 and 5 to cover a considerable portion of this range. The family of graphs for different ternperatures exempliiiedA in Fig. 5 constitute the standard as determined from the average of many composite units having individual units with characteristic at 25 C. as shown in Fig. 6. Knowing the temperature (as is the case with a radio sonde) the humidity is obtained from the frequency-humidity graph for that temperature.

'If the method previously outlined for making and coating the units is carefully followed, it is possible to obtain 'I5-80% of the units which will fall within i2 or 3% of the standard graph.

For use at varying temperatures other than with the radio sonde, a multi-scale indicating instrument may be used as described in connection with Figs. 12 and 13.

4. FLIGHT Rscoans In Fig. 7 there isshown a comparative ight record illustrating the great increase in sensitivity effected by the present invention, as compared to the old style hair-hygrometer. 'Ihis record shows the two hygrometer traces (solid,- electrolvtic; dotted, hair) starting at the same point (about 88% relative humidity), and shows clearly the rapid response and very informative reversals detected by the electrolytic hygrometer, which, except in the case of a prolonged reversal, escape the hair hygrometer. The other half of the chart shows the temperature existing at the respective altitudes (see Fig, 4) from which the corrected values of relative humidity may be obtained as discussed in connection with Fig. 5. In this chart the abscissa values are determined by the audio frequency in terms of which the humidity and temperature are transmitted from the radio sonde. The ordinate represents altitude, each changeover from temperature to humidity being made by the atmospheric pressure operated swicth at a predetermined pressure which may be converted into altitude in feet.

The records shown in Figs. 8 and 8a, further illustrate the very rapid rate of response of the electric hygrometer to sudden humidity changes. Fig. 8 shows the relative speed of response of the electrolytic hygrometer (solid line) and a hair hygrometer (dotted line) at C. when taken from air at 30% relative humidity and placed in at 500 ft. per minute air stream at 66% relative humidity. Both units it will be seen, start with an observed or indicated value or 30% or 36% less than the 66% terminal value. As shown by the curves, the standard time constant P of the electrolytic hygrometer (eleven seconds) is twenty times as fast as that of the hair hygrometer Q, three and one-fourth minutes. 'I'he time constant is the time required for the unit to reach a value indication equal to the product of the original difference, here 36%, multiplied bythe factor: one minus the reciprocal of e, the base of natural logarlthms, which factor has the value 0.63, placing the time constant line PQ' at the value 36-0.63(36) or 36-22.68 which equals 13.32 as shown Similarly, Fig. 8a shows the relative speed of response of the electrolytic unit (solid line) and a hair hygrometer (dotted line) at 24 C. when taken from air at 68% humidity and put in a 500 ft. per minute air stream at 35% humidity, and shows the time constant R for the electrolytic unit, 3 seconds, as against 1 minute for the time constant, S, for the hair hygrometer, in the formula where de1ta=68-35, or 33 and hence the value for RS equals 33-0.63(33), or 12.21, as shown.

It is thus clear that the same 20-to-one improvement pertains both as regards indication of increasing and decreasing humidity.

OPERATING CHARACTERISTICS IN A.C. MEASURING CIRCUITS 1. CIRCUIT AimANoEMEN'r For use other than with the radio sonde the electrical hygrometer should preferably be used in an alternating current circuit as polarization is thereby eliminated. In the case of the radio sonde this is not a serious factor as it does not alter the operation of the hygrometer sufllciently to change the calibration during the period of time (say 1 hour) in which the unit is used. A preferred alternating-current circuit arrangement is that shown in Fig. 9. Here the unit l! is put across about volts 60 cycles alternating current supplied by the adjustable transformer 30, with '166,000 ohms series resistance 3| and the input of a full-wave oxide rectifier 32 in series with it. 'I'he direct-current output of the rectifier is connected to a 0-125 microammeter MA. The microammeter scale may be calibrated in terms of percent relative humidity. The voltage impressed on this circuit arrangement should of course be kept at 100, for which purpose any suitable voltage regulator may be used.

The above circuit arrangement is that 0btained when using the C/ 1,000 scale of a Weston model 764 capacity meter, the electric hygrometer taking the place of the condenser to be measured. This capacity meter may be used as an ohmmeter in this fashion, since it is measuring only ohmic resistance in a nonreactive circuit. This circuit arrangement was adopted because it passes alternating current through the unit, and this current is never greater than 200 microamperes. The current must be kept under this value, as heavy currents would alter the calibration. Information as to the constancy of calibration of a unit if the current is allowed to flow through it continuously is not yet available, although a current of 100 microamperes owing through a unit steadily for several days seemed to have little effect. It is safer, until further data are available, to read humidity by allowing current to ow through the unit only for the time necessary to take a reading, or for control purposes a timing switch may connect the unit into circuit only for a time long enough to affect the control mechanism. If this is not feasible, the hygrometer unit may be left in the control circuit continually by placin-g it in the input circuit of a suitable electron-tube amplifier with the indicating or control device in the output circuit. In such a circuit the current through the unit will .be kept Within safe limits.

The capacity between the two wires running to the humidity unit should be kept as small as possible by separating them. If appreciable, a circuit arrangement described hereinafter may be used.

2. Humour-RESISTANCE CHARACTERISTICS a. Single-unit characteristics Using such a circuit arrangement with the capacity meter calibrated to read in ohms instead of microfarads, the humidity-resistance characteristics shown in Fig. l0 were obtained. Here graphs B to E are for standard units with a 0.25-, 0.5-, 1, and 2.2-percent LiCl coating, respectively. Graph A is that obtained with the polyvinyl-acetate film alone, Without LiCl.

The coatings on these units were so chosen that each unit covers a different range of humidity when varying over the same resistance. Thus, considering a resistance variation from 8 megohms to 75,000 ohms, unit E covers from 10 to 22 percent, unit D 22 to 42 percent, unit C 40 to 60 percent, unit B 50 to 70 percent, unit A 66 to 91 percent. A percentage of LiCl for B to give less overlap would be 0.10 percent, and in this case A may be coated with a somewhat diluted polyvinyl-acetate solution, to cause the unit to register up to 100 percent.

b. Composite-unit arrangement While individual units as described above should'flnd some applications as described hereinafter, it is evident from Fig. that no one unit will cover the full humidity scale. However, by putting the units A to E (Fig. 10) in parallel each with its proper series resistor, as shown in Fig. 2, it is possible to obtain in effect a single unit which covers the humidity range from 10 to 100 percent at room temperatures, with resistance variation confined within the limits of 8 megohms to '75,000 ohms. A satisfactory combination from Fig. 3 is: R1=75,000

ohms, Rz=0.15 megohms, Ra=0.35 megohm.

R4=1 megohm, Rs=3 megohms, with A coated with polyvinyl acetate only, B with 0.25 percent of LiCl, C with 0.5 percent of LiCl, D with 1 percent of LiCl, and E with 2.2 percent of LiCl. The` DESIGNS Foa SPECIAL UsEs A hygrometer of the type described is very flexible in its' application, since it is of the electrically operated type and may be modified to fit specific needs.

If one is interested in a limited humidity range, as for example in the automatic control of humidity, a single-element unit is desirable, as greater sensitivity may be obtained. (See graphs in Fig. 10.) It must be emphasized, however,

that information is not yet available as to theV life of a unit when the control current is allowed to flow through it continually. An electron-tube amplifier or a timing switch may be used, as described above. Replacement units are inexpensive and may be easily plugged into circuit after a given number of hours.

1. DESIGN Foa Usa WHERE A.C. Powna Is AVAILABLE Where it is necessary to run long lines from the humidity-indicating device to the humidity unit, it is desirable to locate the full-wave oxide rectifier 32, as shown in Fig. 9, at the location of the humidity unit and run the direct current output of the rectifier back to the location where the humidity is to be registered. This method is preferred, as the alternating current flowing across the capacity of a long line to the humidity unit would register on the indicating meter.

2. BATTERY-Opnamen UNIT For portable use and in places remote from a 60-cycle alternating current supply a measuring circuit may be used in which the alternating current for operating the unit is obtained from an electron-tube oscillator operating at 60 cycles. It may be powered from small dry cells such as are used in portable radio sets. Such an arrangement is shown in Fig. 14, in which the alternating current is supplied through the unit 33 by oscillator 34 to the input terminals 35 of amplifier unit 36, the output terminals 31 of which are connected to the indicating instrument 33 of conventional design.

3. MUL'rIseALE UNI'I` ron CoNnI'rroNs or WIDE TEM- PEBATUBE VAsrA'rxoNs For registering humidity under a condition of wide variation in temperature, a multiscale indicatcr such as shown in Fig. 12 may be used. Knowing the temperature, the humidity scale for that temperature is used.

A second type' of multiscale arrangement ior use at room temperature is shown in Fig. 13. Here individual units with different sensitivities vare connected to the measuring instrument (Weston model 764 capacity meter set at minimum capacity setting) by means or a five-point switch. Depending upon the humidity, the particular unit may be selected which brings the meter reading within the scale. Thus if the humidity is between 10 and 20 percent, switch contact l and scale I will be used. This arrangement has the advantage of very open scale, allowing greater precision in humidity headings.

Various details of the present invention are described in an article by the present inventor, to which reference is hereby made, entitled An improved electric hygrometer, published as research paper R. P. 1265, in the Journal of Research of the National Bureau of Standards. vol. 23, No. 6, Dec. 1939, page '101, et seq., and to which reference may be had to supplement the foregoing description if desired.

From the foregoing description it will be appreciated that various modifications may be made without departing from the invention exemplied in the preferred embodiments disclosed.

Thus, while polystyrene, polyvinyl-acetate and 1 lithium chloride are preferred constituents found particularly desirable in embodying this invention, in its broader aspects the invention is not limited to the use of these materials.

I claim as my invention:

1. A humidity variable device comprising a support of high electrical resistance non-hygroscopic material, bilar conductors carried thereby, and a thin film of intermixed hygroscopic and binder materials in electrical contact with and overlying said biiilar conductors.

2. A humidity variable device comprising a support of high electrical resistance non-hygroscopic material, bifilar conductors carried thereby, and a film of hygroscopically sensitized polyvinyl acetate in electrical contact with said bifllar conductors.

3. A humidity variable device comprising a support of high electrical resistance non-hygroscopic material, biiilar conductors carried thereby, and a film of polyvinyl acetate hygroscopically sensitized with lithium chloride embedding said bifilar conductors.

4. A humidity variable device comprising a thin highly heat conductive base, a support of high electrical resistance non-hygroscopic material carried by said base, biiilar conductors carried by said support, and a thin hygroscopically sensitive film in electrical contact with and overlying said bifilar conductors.

5.1A humidity variable device comprising a thin-walled aluminum tube, a thin layer of nonhygroscopic electrical insulating material thereon, biiilar conductors wrapped on said insulating material, and a thin'hygroscopically sensitive film in electrical contact with and overlying said biillar conductors.

6. A humidity variable device comprising a thin highly heat conductive base, a thin supporting layer of polystyrene resin thereon, bifilar conductors supported by said layer, and a hygroscopically sensitive film in electrical contact with said bifllar conductors.

'7. A humidity vvariable device comprising a thin aluminum base, a thin supporting layer of polystyrene resin thereon, bifilar conductors supported by said layer, and a hygroscopically sensitive nlm in electrical contact with said biillar conductors.

8. A humidity variable device comprising a thin walled aluminum tube, a supporting layer of polystyrene adhering to said tube, bilar conductors wrapper on said polystyrene and slightly seated therein, and aillm of polyvinyl acetate hygroscopically sensitized with lithium chloride adhering to said conductors and said polystyrene support.

9. In a humidity variable device of the type described, a non-hygroscopic polystyrene support, binlar conductors carried thereby, and a hygroscopically sensitive nlm in electrical contact with said conductors.

10. In a humidity variable device oi the class described, a thin aluminum base having a nonhygroscopic polystyrene resin coating thereon, biiilar conductors seated slightly in said polystyrene resin, and a hygroscopically sensitive nlm overlying said conductors and said polystyrene coating and in electrical contact with said conductors.

11. In an electrical hygrometer circuit, a humidity responsive element comprising bifilar conductors electrically connected by a hygroscopically sensitive lm, a source of predetermined alternating current, a rectiiler, a circuit connecting said current source and said element with the input of said rectifier to supply thereto an alternating current controlled by said element, and means for measuring the direct-current output of said rectiiier as a measure oi the humidityindication of said element.-

12. A humidity responsive system comprising an electron tube generator oi alternating current, an electron tube ampliner, a humidity responsive element comprising a supporting area oi high electrical resistance non-hygroscopic material, binlar conductors carried by said area and a hygroscopically sensitive iilm in electrical contact with said bilar conductor, a control 'circuit for said ampliiler powered by the output of said generator,l said humidity responsive element being connected in said control circuit, and translating means operated by the output of said amplier.

13. A humidity variable device comprising a support of high electrical resistance non-hygroscopic material, a pair of spaced conductors carried thereby and a thin nlm of intermixed hygroscopic material and amorphous material pervious to moisture in electrical contact with and overlying said pair oi conductors.

14. In a humidity device having two electrical terminals: a plurality of elements; each element comprising a supporting area o! highly non-hygroscopic material having a high electrical resistance, two separated electrical conductors carried by said area, and a hygroscopcally sensitiva illm in contact with said conductors and said supporting area; the hygroscopic nlms on said respective elements containing binder-material and hygroscopic salt in different plOROrtions, respectively; said device further comprising electrical resistors of different values and having one conductor oi each element connected to one of its said electrical terminals and the other conductor of each element connected through one or more of said resistors of diierent value to the other of its said electrical terminals, and having the values oi' resistance in series with its respective elements greater, the greater the amount of hygroscopic salt in the nlm of the respective element.

15. In a humidity device having two electrical terminals; a plurality of elements; each element ,comprising a supporting area of highly non-hygroscopic material having a high electrical resistance, two separated electrical conductors carried by said area, and a hygroscopically sensitive 111m in contact with said conductors and said supporting area; the hygroscopic nlms on said respective elements containing binder material and hygroscopic salt in diii'erent proportions, respectively; said device further comprising an electrical resistor and having one conductor of each element connected to one of its said electrical terminals, the other conductor of one ot its elements connected to the second of its said electrical terminals, and the other conductor of another of its elements connected through said resistor to the second of its said electrical terminals.

i6. In a humidity device having two electrical terminals; a plurality of elements; each' element comprising a supporting area' of highly non-hygroscopic material having a high electrical resistance, two separated electrical conductors carried by said area, and a hygroscopically sensitive film in contact with said conductors and said supporting area; the hygroscopic nlms on said respective elements containing a ic binder fortined, save for one element, with dii'- ierent amounts of hygroscopic salt; said device further comprising electrical resistors of diil'erent values and having one conductor oi each element connected to one of its said electrical terminals and the other conductor of each element connected through one or more of said resistors of diierent value to the other of its said electrical terminals, and having the values of in series with its respective elements greater, the greater the amount of hygroscopic salt in the nlm of the respective element.

FRANCIS w. DUNMORE. 

